If you're the kind of guy who likes to miss at 400 yards with a .300 mag, this round will allow you to miss at ranges over 500 yards!
Seriously, not one riflemen in ten is really competent beyond 300-350 yards. Why would someone want to burn more powder to throw a .308 slug a little flatter?
Unless you've attended the sniper school at Quantico, the .30/06 will do everything you will ever need.